The presence of sulphur compounds in gas flows is undesirable, because of their toxicity and their smell. Hydrogen sulphide (H.sub.2 S) is a harmful compound that is frequently present in gas flows, especially in biogas originating from anaerobic waste treatment. Sulphur dioxide is another noxious sulphur compound that is present in gas flows resulting from the combustion of fossil fuels. Other harmful sulphur compounds that may be present in gas flows include sulphur trioxide, carbon disulfide, lower alkyl mercaptans etc. Gaseous effluents containing these sulphur compounds must therefore be purified, before they can be discharged into the atmosphere.
A process for removal of H.sub.2 S from a gas flow by scrubbing with a washing liquid and subsequently oxidising the absorbed sulphide is known from European patent application 229,587. According to that process, sulphide absorbed in the washing liquid is oxidized non-biologically, in the presence of a catalyst, to products including elemental sulphur which is separated from the system. However, loss of catalyst occurs in such a process, which is an environmental drawback and which increases costs. The known process is also relatively expensive because oxidation takes place using pressure.
A process frequently used is washing biogas with an aqueous liquid having an increased pH, typically a pH of about 11. This increased pH may be adjusted by the addition of sodium hydroxide or other alkalis. Such processes are known for example from European patent application 229,587. A drawback of scrubbers of this type is their high consumption of chemicals, resulting in relatively high operational costs. The price of sodium hydroxide has been increasing strongly recently as a result of a reduced production of chlorine. For many industries, savings in sodium hydroxide will therefore become important. Another disadvantage of this process is that it results in aqueous effluent contaminated with sulphide. According to NL-A-8801009 spent washing liquid from H.sub.2 S removal from gases can be regenerated by subjecting it to sulphur-oxidizing bacteria in the presence of oxygen.
Another method consists in mixing the biogas with air or oxygen then conveying it into an oxidation reactor, wherein the sulphide is converted to sulphur, as is known from European patent application 224,889. A drawback of this method is that the reactor becomes rather expensive, because the mixture of biogas and oxygen is explosive, requiring safety precautions. The reactor should also be rather large because the conversion rate is strongly reduced by the low concentration of oxygen which is allowed in connection with the gas effluent requirements (explosion standards).
Known processes for the removal of sulphur dioxide from gaseous effluents involve washing the gas flow with an acidic aqueous washing liquid having a pH which is typically below 5.8. The dissolved SO.sub.2 is usually oxidised and separated as calcium sulphate (gypsum).